1. Field of the Invention
The present invention herein relates to evaluation of performance and petrophysics of subterranean naturally-fractured hydrocarbon reservoirs.
2. Description of the Related Art
In evaluating subsurface reservoirs of interest for the presence and production of hydrocarbons, two types reservoirs are commonly encountered. The first and more often encountered type of subsurface hydrocarbon reservoir is what has become known as a conventional or non-fractured reservoir. The formation rock has an inherent porosity and permeability because of its lithological and textural composition, with connected “conventional reservoir” pores, and hydrocarbon fluids can therefore accumulate in the rock. The other type of reservoir is what is known as a microfractured reservoir, in which porosity and/or permeability is not inherent from the time of rock deposition. The two types of reservoirs are different types of reservoirs with different pore space types, requiring different techniques for characterization.
Microfractured reservoirs are dependent on the presence of fractures in the formation rocks for their porosity and permeability. Pore space in microfractured reservoir formation rock is in effect controlled by, and formed as a result of, the presence of connected microfractures. Microfractures are different from connected, conventional reservoir pores in terms of their nature, origin and their impact on reservoir performance. Microfractures are cracks that occur in the rocks due to deformation under natural earth stresses. They are distinctive in having planar or semi-planar shapes, and with clearly defined orientation.
In evaluating actual or projected performance of certain hydrocarbon reservoirs, particularly in rock formations of low porosity and permeability known as tight gas reservoirs, the presence of microfractures in formation rock has played an important role. Microfractures in rock formations are typically very small, on the order of ten microns to a millimeter scale. Accordingly, microfractures cannot, so far as is known, be readily characterized by visual methods using core samples. Further, microfractures are below the resolution levels of borehole images.
It has been conventional to characterize the presence of microfractures in core samples using either optical or electron microscopy techniques on thin sections taken from the core samples. Results obtained from these methods gave only rough assessments, and that often inaccurate ones, of the fracture-related physical characteristics of the formation rocks of interest. These techniques were also very time consuming and expensive. Further, they did not permit precise or direct assessment of the microfractures' contribution to rock petrophysics.